Soil-borne pathogenic fungi causes damping off, root-rot, crown-rot and neck-rot in a wide variety of seedling crops. Among these pathogenic fungi are fungi of the genera Rhizoctonia, Pythium, Sclerotium, Phytophotora and Fusarium. These fungi are capable of attacking and causing extensive damage to many common and commercially important crops, such as wheat, beans, tomatoes, cotton, peanuts, potatoes, melons, lettuce, ornamental flowers and others. Fusarium spp. in particular has an extraordinarily wide host range and is capable of attacking many commonly grown and commercially important crops.
F. roseum "Culmorum", for example, is a cosmopolitan pathogen occurring on an extremely wide range of host plants, including wheat. F. roseum "Culmorum" is of economic significance, since it is a causal agent of cortical rots and pre- and post emergence blight.
F. oxysporum spp. are also commercially significant pathogens. F. oxysporum f. sp. radicis-lycopersici causes crown rot in tomatoes. Unlike other strains of F. oxysporum, F. oxysporum f. sp. radicis-lycopersici is a causal agent of stem rot. Other destructive F. oxysporium spp. are f. sp. vasinfectum which attacks crops including cotton, and f.sp. melonis which attacks melon crops. These pathogens are world-wide agricultural problems.
Chemical fungicides have been widely used to control these soil-borne pathogens. The use of such chemicals is expensive and may result in ecological damage and in the increased incidence of occupational diseases. One particular problem in controlling Fusarium spp. infestation in crops is the fact that most chemical fungicides are not sufficiently effective. Only systemic fungicides result in some positive result, and even those are only partially effective. The method most commonly used today is soil fumigation with methyl bromide and chloropicrin, at high application rates prior to planting. A promising alternative to such chemical control is the biological control of soil-borne plant pathogens by naturally-occurring microorganisms. These biological control agents may be used alone or in conjunction with lesser amounts of chemical fungicides.
The use of antagonistic microorganisms in controlling plant pathogenic fungi has been the subject of extensive research. A large part of this research has been concerned with myco parasitism, the parasitism by one fungus of another. One of the most frequently studied mycoparasites in relation to biological control is the genus Trichoderma. (Y. Elad et al., 1982, Can. J. Microbiol. 28: 719-725, I. Chet and R. Baker, 1981, Phytopathology 71: 286-290; M. N. Schroth and J. G. Hancock, 1981, Ann. Rev. Microbiology 35: 459-463; Y. Elad et al., 1981, Plant Disease 65: 675-677; Y. Elad, et al., 1980, Phytopathology 70: 119-121; I. Chet et al., 1979, in B. Scippers and W. Gams, eds, "Soil Borne Plant Pathogens", Academic Press, NY, NY; Y. Hadar et al., 1979, Phytopathology 69: 64-68; C. Dennis and J. Webster, 1971, Trans. Br. mycol. Soc. 57(3), 363-369).
Species or strains of Trichoderma may be differentially antagonistic to different species of fungi. (H. D. Wells et al., 1972, Phytopathology 62: 442-447). Such differences in antagonism have been found both within and between species of Trichoderma (D. K. Bell et al., 1982, Phytopathology 72: 379-382).
It was thus found that the species T. harzianum and T. hamatum showed high fungicidal activity, especially against Sclerotium rolfsii and Rhizoctonia solani. T. hamatum was also found to be effective against Phythium spp.
In addition to such differences in antagonism, it has been determined that the environment in which the interaction occurs also affects the degree of biological control. Antagonism in culture is often not reproducible in the complex environment present in the soil under greenhouse or field conditions. (M. N. Schroth and J. G. Hancock, 1981, Ann. Rev. Microbiol. 35: 453-76). As a result, tests made under non-soil culture conditions are not truly indicative of the potential for use of the Trichoderma isolate as a biological control agent.
Due to the significant differences in antagonism of Trichoderma isolates to various pathogens under different environmental conditions, researchers have concentrated their efforts on searching for Trichoderma antagonists against specific disease causing plant pathogenic fungi.
One strain of Trichoderma harzianum, T. harzianum Rifai T-315 (ATCC No. 20671) has been found to be effective in combating several pathogenic fungi, and exhibits some antagonistic activity against Fusarium spp. (co-owned and copending U.S. Ser. No. 588,950, filed Mar. 31, 1984). As in the case of chemical fungicides, however, T. harzianum T-315 (ATCC No. 20671) is only partially effective in protecting crops from Fusarium spp. Thus the need for specific and highly effective agents which are antagonistic to Fusarium spp. has heretofore remained unmet.